P–T Conditions of Generation of the Pea Negra Anatectic Complex, Central Spain*

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Petrology, Vol. 6, No. 6, 1998, pp. 555–563. From Petrologiya, Vol. 6, No. 6, 1998, pp. 606–614.

INTRODUCTION

The Pe a Negra anatectic complex (PNAC) is located in the middle-western part of the Avila batholith, in central Spain (Fig. 1) (Bea and Pereira, 1990; Pereira, 1992; Pereira and Bea, 1994). It crops out over 350 km², and it is made up of high–medium grade metamorphic rocks, mainly migmatites, and anatectic products derived from the melting of those migmatites during the Hercynian orogeny. Generation and coexistence of the different products is mainly the result of the locally composition variation of a highly heterogeneous source. A fertile lithology is the main feature to take into account, although the presence of shear structures, controlling the role of volatile elements, is a very important factor in triggering the partial melting of the migmatites (Pereira and Shaw, 1996, 1997).

Migmatites have a nonvariable mineralogy, characteristic of low pressure conditions: quartz + plagioclase + alkali feldspar + biotite + cordierite ± sillimanite as main phases. Ilmenite, apatite, zircon, pyrite, chalcopy- rite and tourmaline are common accessory minerals.

This paragenesis does not contain an association that can be used as a good thermobarometric sensor. How- ever, there is a restricted and narrow band of migmatites, close to the high part of the complex, where we have identified garnet as an accessory phase (Pereira, 1992, 1993). This fact will be crucial when applying some geothermometric calculations, calibrated by different authors on the pairs cordierite–garnet (Bhatta- charya et al., 1988), biotite–garnet (see Spear and Pea- cock, 1990), and on the association garnet–plagioclase–sillimanite–quartz to calculate pressure conditions (Hodges and Spear, 1982).

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The purpose of this paper, therefore, is the study of pressure and temperature conditions under which the rocks from the PNAC were generated and to compare these results with the ones obtained by other authors for the generation of similar complexes.

THE SAMPLES

Mesocratic migmatites are the most abundant material in the PNAC, and sheets of granodiorite and small lens-shaped bodies of cordierite leucogranite can be found interlayered with them (Pereira, 1992; Pereira and Bea, 1994; Pereira and Shaw, 1996, 1997). These migmatites are high-grade, mostly neosomatic, and can be called diatexites (in the sense of Mehnert, 1987).

They are made up of: (1) a granodioritic leucosome, with hypidiomorphic texture, containing quartz, plagioclase, alkali feldspar, cordiente, and biotite as main phases; and (2) a restitic melanosome, which appears as small enclaves or as schlieren within the leucosome, and it is made up by sillimanite, cordierite, and biotite, with very abundant ilmenite and very rare alkali feldspar.

Chemical and mineralogical composition of migmatites is identical in every part of the PNAC, but garnet is only found in a specific outcrop, where it appears as subhedral big crystals (1 cm in size) with poikilitic textures (Fig. 2). Garnet has a tendency for the leucosome, although composition is the same as the few found in melanosome (Table 1; Fig. 3).

ANALYTICAL METHOD

Electron microprobe analyses were done for major elements on biotite, cordierite, garnet, sillimanite, muscovite, feldspar, ilmenite, and sulfides. Facilities used

P–T Conditions of Generation of the Pe a Negra Anatectic Complex, Central Spain*

M. Dolores Pereira

Depto. de Geología, Facultad de Ciencias, Universidad de Salamanca, 37008 Salamanca, SPAIN;

e-mail: [email protected] Received April 21, 1998

Abstract—Geothermobarometry has been applied on materials from the Pe a Negra anatectic complex using assemblages containing garnet. The used calibrations show that the climax of anatexis took place at 750°C and 4 kbar. Retrograde metamorphism led to minimum temperature conditions of about 500°C and a pressure between 1 and 2 kbar. Garnet is a very rare mineral within the complex and seems to have formed through the incongruent melting of biotite during the thermal paroxysm. Afterwards, retrogression of the garnet produced a rim of biotite plus cordierite; these are less magnesian than normal prograde biotite and cordierite. The obtained results coincide with published data by different authors for areas nearby, which are always near the anatexis isograd.

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* This article was submitted by the author in English.

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556 DOLORES PEREIRA

Avila Avila Avila

Bejar Bejar Bejar

Guljuelo Guljuelo Guljuelo

HESPERIAN MASSIFHESPERIAN MASSIFHESPERIAN MASSIF

LEGEND Tertiary sediments Peña Negra complex Metamorphic rocks Hercynian Granitoids Fault

N

0 10 20

km

Fig. 1. Location of the Pe a Negra anatectic complex within the Avila batholith (Central Iberia, Hercynian massif).n˜

2 1

4

3

Fig. 2. Characteristic texture of garnet in migmatites from Pe a Negra complex (see text).

(1) Garnet; (2) cordierite; (3) biotite; and (4) sillimanite (fibrolite). Scale bar: 1 mm.

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P–T CONDITIONS OF GENERATION 557

were CAMEBAX microprobes at the University of Oviedo (Spain) and Hannover (Germany). Similar analytical programs were used for the silicates and the parameters were: accelerating voltage: 15.00 kV; fila- ment tension: 8.00 kV; image energy: 10 nA.

As standards were used (1) corundum, (2) hematite, (3) periclase, (4) TiMn, (5) orthoclase, (6) albite, (7) wollastonite, (8) andradite, and (9) apatite. Counting conditions were adjusted to obtain a precision better than ~0.5 % in all elements.

Tables 1–4 show the analyses obtained for garnet, cordierite, biotite, and plagioclase, respectively.

CHEMISTRY OF THE GARNET

Garnet from the PNAC migmatites has an almand- ine composition with a high content of spessartine (up to 8%). MnO is concentrated preferentially in the rim, the core is slightly depleted in this component (Fig. 3).

Profiles are similar to those described by several authors in rocks from sillimanite–alkali feldspar metamorphic conditions. However, garnet from the litera- ture shows a large core without zoning, and all the zoning is concentrated in the rim (e.g., Tracy et al., 1976;

Table 1. Analyses of garnet from Pe a Negra migmatites

Components 1 2 3 4

rim mid core rim mid rim mid core rim mid core

SiO₂ 36.73 37.03 37.08 36.38 37.33 36.79 36.33 37.02 36.23 36.83 36.46

TiO₂ 0.04 0.04 0.02 0.03 0.04 0.01 0.04 0.00 0.00 0.00 0.04

Al₂O₃ 20.78 20.58 20.68 20.53 21.00 20.75 20.83 20.98 20.77 20.67 20.66

FeO 36.16 36.91 35.92 36.72 35.17 34.24 35.03 34.08 34.29 35.46 35.05

MgO 1.18 1.90 2.56 1.18 3.41 1.88 2.03 3.06 0.61 1.71 1.70

MnO 5.76 4.29 3.75 5.22 3.13 4.91 4.54 3.81 6.59 5.00 4.89

CaO 0.71 0.81 0.74 0.66 0.83 0.78 0.82 0.80 0.69 0.71 0.79

Na₂O 0.01 0.00 0.01 0.00 0.00 0.02 0.04 0.06 0.09 0.08 0.09

K₂O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.02 0.01 0.00

Total 101.4 101.5 100.8 100.7 100.9 99.38 99.66 99.82 99.29 100.47 99.68 Structural formulae based on 12 oxygens

Si 2.98 2.98 2.99 2.97 2.98 3.00 2.97 2.99 2.99 2.99 2.98

Al 1.98 1.95 1.96 1.97 1.98 2.00 2.00 2.00 2.02 1.98 1.99

Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Fe 2.45 2.49 2.42 2.51 2.35 2.34 2.39 2.30 2.37 2.41 2.40

Mg 0.14 0.22 0.30 0.14 0.40 0.22 0.24 0.36 0.07 0.20 0.20

Mn 0.39 0.29 0.25 0.36 0.21 0.34 0.31 0.26 0.46 0.34 0.33

Ca 0.06 0.07 0.06 0.05 0.07 0.06 0.07 0.06 0.06 0.06 0.06

Na 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01

K 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

X_Fe 0.80 0.80 0.79 0.81 0.77 0.78 0.79 0.76 0.79 0.79 0.79

X_Mg 0.04 0.07 0.10 0.04 0.13 0.07 0.08 0.12 0.02 0.06 0.06

X_Mn 0.13 0.09 0.08 0.11 0.07 0.11 0.10 0.08 0.15 0.11 0.11

X_Ca 0.02 0.02 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02

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6 5 3 4 2 38 37 36 35 4 3 2 1 1.0 0.9 0.8 0.7 0.6 MnO, %FeO, %MgO, %CaO, %

1 mm

GARNET 1 GARNET 2

rim core rim rim core rim

Fig. 3. Zoning profiles for MnO, FeO, MgO, and CaO in garnets from migmatites in the Pe a Negra complex.n˜

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558 DOLORES PEREIRA

Yardley, 1989). Garnets from the PNAC migmatites show a continuous zoning from core to rim, although some irregularities are found in the middle part of the largest crystals (Fig. 3).

The FeO profile is similar to the MnO one: concentration of these components increases toward the rim.

Regarding MgO, there exists a depletion toward the rim, although in some cases, there are zones between showing an enrichment in comparison to the core. The depletion of MgO toward the rim can be explained by cordierite generation at the expense of garnet.

In general, a reverse zoning is observed on the garnets from the PNAC migmatites related to its resorp- tion and generation of cordierite and biotite at their expense.

GEOTHERMOBAROMETRIC STUDY There is a large collection of references dedicated to the calibration of geothermometers based on the same mineralogical association that is found in the materials used for the present study (e.g., Perchuk, 1967, 1969, 1970a, 1970b; Hensen and Green, 1973; Currie, 1974;

Holdaway and Lee, 1977; Perchuk, 1977; Ferry and Spear, 1978; Perchuk, 1981; Hodges and Spear, 1982;

Perchuk and Lavrent’eva, 1983; Ganguly and Saxena, 1984; Indares and Martignole, 1985; Bhattacharya et al., 1988; Spear and Peacock, 1989). However, only few were used to calculate P and T conditions for the PNAC (Table 5) avoiding those that gave aberrant results.

More specifically, for the pair garnet–cordierite, the algorithm of Bhattacharya et al. (1988) was used, and for the pair garnet–biotite, Ferry and Spear (1978), Hodges and Spear (1982), Ganguly and Saxena (1984), Table 2. Analyses of cordierite from Pe a Negra. Crystal 3

is a cordierite coexisting with garnet. In the structural formulae, ΣT, ΣM, ΣCh represent the sum of tetrahedric, octahedric and channel cations respectively. mg# = Mg/(Fe + Mn + Mg)

Com-

ponents 1 2 3 4 5

SiO₂ 47.64 48.25 48.17 48.53 47.83

TiO₂ 0.00 0.02 0.01 0.00 0.00

Al₂O₃ 32.04 32.85 31.64 32.27 32.86

FeO 11.95 10.41 13.08 11.58 10.77

MgO 6.10 6.50 5.34 6.09 6.08

MnO 0.23 0.22 0.33 0.27 0.26

CaO 0.00 0.01 0.00 0.00 0.01

Na₂O 0.18 0.39 0.15 0.08 0.25

K₂O 0.00 0.01 0.00 0.00 0.00

Total 98.14 98.66 98.72 98.82 98.06 Structural formulae based on 18 oxygens

Si 5.004 5.004 5.053 5.045 4.998

Al 3.966 4.014 3.910 3.953 4.046

ΣT 8.970 9.019 8.963 8.997 9.043

Ti 0.000 0.002 0.001 0.000 0.000

Fe 1.050 0.903 1.147 1.006 0.941

Mg 0.954 1.004 0.835 0.943 0.946

Mn 0.021 0.019 0.030 0.024 0.023

ΣM 2.025 1.928 2.012 1.974 1.910

Ca 0.000 0.001 0.000 0.000 0.001

Na 0.037 0.078 0.031 0.016 0.051

K 0.000 0.001 0.001 0.000 0.000

ΣCh 0.037 0.081 0.032 0.016 0.052

Σcat. 11.032 11.027 11.007 10.987 11.005

mg# 0.471 0.521 0.415 0.478 0.495

n˜ Table 3. Analyses of biotite from Pe a Negra migmatites.

Crystals 1 and 2 are in contact with garnet Com-

ponents 1 2 3 4

SiO₂ 34.40 34.60 35.03 34.72

TiO₂ 2.34 2.30 2.43 2.39

Al₂O₃ 19.80 19.46 18.29 19.13

FeO 22.67 23.13 21.31 20.62

MgO 5.82 5.90 8.06 8.10

MnO 0.21 0.06 0.20 0.22

CaO 0.01 0.04 0.00 0.00

Na₂O 0.23 0.22 0.08 0.20

K₂O 9.03 9.21 9.98 8.86

Total 95.51 94.92 95.38 94.24

Structural formulae based on 11 oxygens

Si 2.681 2.692 2.705 2.686

Al(IV) 1.319 1.308 1.295 1.314

Al(VI) 0.500 0.477 0.369 0.430

Ti 0.137 0.135 0.141 0.139

Fe 0.676 0.684 0.927 0.933

Mg 1.477 1.505 1.376 1.334

Mn 0.014 0.004 0.013 0.014

ΣM 2.804 2.803 2.825 2.851

Ca 0.002 0.004 0.000 0.000

Na 0.036 0.033 0.012 0.030

K 0.898 0.914 0.983 0.874

ΣA 0.935 0.951 0.995 0.904

0.686 0.688 0.597 0.588

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Fe Fe+Mg ---

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P–T CONDITIONS OF GENERATION 559

Perchuk and Lavrent’eva (1983), Indares and Marti- gnole (1985), and Spear and Peacock (1990). The geobarometer used was the one by Hodges and Spear (1992), using the association garnet–sillimanite–plagioclase–quartz.

GEOTHERMOMETRY Cordierite–Garnet Pair

Some of the calibrations considering the pair cordierite–garnet did not offer a reasonable result for the metamorphic climax. That is probably due to the lack of spessartine component of the garnet used for the calibration, which in garnet from the PNAC can be very high (see above) with a high and constant enrichment toward the crystal rim (Table 1).

Only the thermometer calibrated by Bhattacharya et al. (1988) has been applied, yielding reasonable temperature values (Table 5):

(1) The range of temperature values is 513–741°C.

T has been calculated for a constant P of 3 kbar, as this variable does not affect the temperature calculation.

(2) Depletion of MgO toward the rim has been explained as a consequence of cordierite generation (Grant and Weiblen, 1971; Tracy et al., 1976; Hollister, 1977; Tracy, 1982; Pereira and Bea, 1994).

(3) It can be observed that when using rim composition, the highest X_Fe/X_Mg values are obtained, together with the lowest temperatures. However, when using garnet core composition, where the lowest values for X_Fe/X_Mg are found, we get the maximum values of T, for a constant P = 3 kbar. It is highly recommended to use core analyses when dealing with zoned minerals to get equilibrium temperatures, so we can conclude that the value around 740°C is the temperature for the equilibrium crystal–melt.

Biotite–Garnet Pair

Distribution of FeO and MgO between the garnet core and biotite from the matrix is more appropriated to estimate temperature conditions in prograde metamorphism (Tracy et al., 1976; Chipera and Perkins, 1988).

This is due to the high diffusion rate of biotite in comparison to garnet, in the way that while zoning can be determined on the latter, biotite in contact with garnet does not show any zoning, as diffusion has deleted it (Robinson et al., 1982; Spear, 1989).

Several thermometers have been applied, using the program developed by Spear and Peacock (1990) on the biotite–garnet pair. Analyses from core, middle, and rim have been used, attending to the garnet zoning, from leucosome and melanosome of migmatites of the PNAC. Although there is agreement when using most of them, thermometers calibrated by Perchuk and Lavrent’eva (1983) and Indares and Martignole (1985) gave much lower values than the others. Temperatures obtained when using core or rim analysis are similar to the ones obtained with the calibrated method by Bhat- tacharya et al. (1988) on the garnet–cordierite pair.

Therefore, we can accept that the maximum T values obtained (750°C for constant P = 3 kbar) represent the climax of metamorphism.

GEOBAROMETRY

Due to the mineralogical composition of the PNAC, only the barometer calibrated by Hodges and Spear (1982) has been used on the association garnet–plagioclase–sillimanite–quartz. Composition of plagioclase in the migmatites is rather constant: core An_20–26, rim An_11–18. Chemical data of plagioclase can be found on Table 4.

Although in temperature calculation, we used a constant P of 3 kbar because of the independence of the Table 4. Analyses of plagioclase from Pe a Negra

Com-

ponents 1 2 3 4

SiO₂ 63.80 62.50 62.61 63.17

TiO₂ 0.00 0.00 0.01 0.00

Al₂O₃ 22.66 23.39 24.64 22.83

FeO 0.01 0.06 0.05 0.04

MgO 0.02 0.00 0.01 0.00

MnO 0.01 0.03 0.05 0.03

CaO 4.22 4.85 5.15 4.11

Na₂O 9.10 8.62 7.89 8.93

K₂O 0.20 0.20 0.21 0.17

P₂O₅ 0.10 0.14 0.14 0.20

Total 100.12 99.79 100.76 99.48

n˜ Table 5. Results from the application of several geother- mometers (°C) for an invariable P of 3 kbar

Ther-

mometer Core Middle Rim Observation

1 741 622 513

2 785 643 488

3 795 652 496

4 764 658 535 dWMn = 3000 cal

5 678 610 525

6 676 555 430

7 782 644 494

Note: Cordierite-garnet pair: 1—(Bhattacharya et al., 1998). Bi- otite-garnet pair: 2—(Ferry and Spear, 1978); 3—(Hodges and Spear, 1982); 4—(Ganguly and Saxena, 1984); 5—(Perchuk and Lavrent’eva, 1983); 6—Indares and Martignole, 1985); 7—(Ferry and Spear, 1990).

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560 DOLORES PEREIRA thermometers on this variable, a difference has been

observed when calculating pressure for equilibrium conditions. For garnet core analyses, the maximum value of 4 kbar was obtained, which drops down to 1–

2 kbar when using the rim analyses.

Also using the middle garnet analyses, we have rep- resented all the values on a P–T diagram, obtaining a retrogressive path, from the equilibrium garnet–melt (P ≈ 4 kbar and T ≈ 750°C) to the end of retrogression of the garnet (P ≈ 1–2 kbar and T ≈ 500°C) (Table 6, Fig. 4).

On Fig. 4, the triple point is the one from Bohlen et al. (1991): T ≈ 530°C; P ≈ 4.2 kbar. H₂O concentration curves are from Johannes and Holtz (1990); muscovite (Ms) and biotite (Bt) melting curves are from Petö (1976) and Le Breton and Thompson (1988), respectively. Relevant mineral equilibrium regarding paragenesis in the PNAC materials are:

(a) ms + qtz = sil + Kfs + vapor, Chaterjee and Johannes (1974) modified by Holdaway and Lee (1977);

(b) bt + sil + qtz = crd (M = 0.5) + Kfs + vapor, bt + sil + qtz = crd + grt + Kfs + vapor, and

grt + sil + qtz + vapor = crd (M = 0.5), M = Mg/(Mg + Fe) (Holdaway and Lee, 1977); nnd

(c) bt + crd + qtz = Kfs + grt from Thompson (1982).

P–T TRAJECTORY

We have not found preanatectic parageneses within the PNAC, but in areas nearby, there are very similar materials, where the anatexis isograde has not been reached (see Franco, 1980; Beetsma, 1995), and the association biotite + sillimanite + quartz can be found

to be stable. This point is taken as the first reference in the P–T trajectory of the metamorphism (point A in Fig. 5).

Garnet is a very rare component of migmatites from PNAC (see above). It has only been identified in a very restricted area, made of migmatite of schlieren facies.

The first question to answer is whether there is a chemical difference between migmatites with and without garnet, specially regarding the aluminum index (ASI) and X_Fe value. Chemistry of all migmatites is virtually the same (Pereira, 1992), so we have to accept that generation of garnet was not due to a compositional pecu- liarity, but to the change of P–T conditions that allowed this material to reach one of the equilibrium curves that represent the garnet generation reaction. From the geothermobarometric study, we can conclude that garnet was generated through the incongruent melting of biotite in the presence of sillimanite during the meta- morphic paroxysm of the PNAC (point B in Fig. 5).

The scarce garnet one can find in the migmatites from the PNAC always presents the same texture (Fig. 2), being surrounded by cordierite and biotite.

These biotite and cordierite have a different composition (less magnesium, more iron) than the other biotite and cordierite coexisting in the same samples (Tables 1–3).

Besides, garnet zoning profiles regarding FeO and MnO are always reverse, implying the effect of retrograde metamorphism. Retrogression of garnet was produced following a continuous reaction where cordierite and biotite were formed at the expense of garnet (point C in Fig. 5) (Pereira, 1992, 1993; Pereira and Bea, 1994):

alkali feldspar + garnet₁ + biotite₁ + cordierite₁ = garnet₂ + biotite₂ + cordierite₂ + quartz.

kyanite

andalusite

ms + qtz

bt + sil + qtz M

sillimanite

bt + sil + qtz Kfs + sil +

vapor

crd (M = 0.5) crd (M = 0.5) + Kfs + vapor

Kfs + grt +

melt bt + crd + qtz

450 550 650 750

0 2 4 6 8 P, kbar

T, °C

10 8 6 4

B

grt + sil + qtz + vapor 2 crd + grt + Kfs +

vapor

Fig. 4. P–T evolution in the Pe a Negra complex defined through the analytical study of garnet profiles: from the climax of meta- morphism to the retrogression conditions. Abbreviations as in (Kretz, 1983).

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P–T CONDITIONS OF GENERATION 561

From this reaction it is obtained:

(1) Garnet₂ is more ferrous and more manganesifer- ous than garnet₁. Garnet₂ is represented by the rim of the largest garnet crystals.

(2) Biotite₂ is less magnesian and less titaniferous than biotite₁, because it is generated, at least partially, at the expense of garnet.

(3) Cordierite₂ is less magnesian and less mangane- siferous than cordierite₁, because it is generated partially at the expense of garnet, and this mineral is get- ting more and more avid for the coordination of Mn.

GEODYNAMIC IMPLICATIONS

Figure 5 shows the conditions under which the PNAC was generated. The same clockwise P–T path has been described in other Hercynian areas nearby (Gil Ibarguchi and Martínez, 1982; Barbero, 1995;

Yenes, 1996), responsible for the generation of important volumes of peraluminous granitoids. It has been interpreted as indicative of brief intervals of rapid tectonic thickening, followed by exhumation of materials, as explained by Hollister (1994). Tectonic thickening was the result of thrusting during the compression tak- ing place during the first phases of the Hercynian orogeny, and as a response of these event, granitic products would have been produced from partial melting of migmatites. An extensional collapse of the series at the end of the orogeny had an effect in decreasing pressure to produce another stage of anatexis, generating low melt fraction granites from a source already depleted in fertile components.

SUMMARY

From the geothermobarometric studies on the mig- matites from the Pe a Negra anatectic complex, a P–T trajectory is obtained indicating that the metamorphic paroxysm took place under maximum conditions of T ≈ 750°C and P ≈ 4 kbar. Afterward, a drop of conditions took place and a retrometamorphism was achieved under minimum conditions of T ≈ 500°C and P ≈ 1–2 kbar.

The observed P–T path in this region is very similar to the one described for areas nearby, using the same mineral associations (Gil Ibarguchi and Martínez, 1982; Barbero, 1995; Yenes, 1996), indicating a stage of isobaric heating, followed by a stage of decompression and cooling, which is reflected by the retrogression of garnet. Jamieson (1991) described these trajectories as characteristic of compressive orogens, and corre- spond to: (I) the isobaric heating during the thermal stage that follows the thickening of the crust and (II) decompression and cooling due to the late exhumation stage.

ACKNOWLEDGMENTS

The author thanks T. Gerya and A. Graphchikov for revision of the manuscript.

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n˜

ms + qtz

C 0

2 4 6 8 P, kbar

450 550 650 750 T, °C

Kfs + sil + vapor

bt + sil + qtz

crd (M = 0.5) kyanite

andalusite

M B

A

B

sillimanite bt + sil + qtz crd (M = 0.5) + Kfs + vapor

Kfs + grt +

melt bt + crd + qtz

crd + grt + Kfs + vapor

grt + sil + qtz + vapor

10 8 6 4

2

Fig. 5. Metamorphic evolution of the Pe a Negra complex.

M and B curves are muscovite (Petö, 1976) and biotite (Le Breton and Thompson, 1988) melting respectively. M = Mg/(Fe + Mg) in cordierite. Dashed lines: water concentration curves are from Johannes and Holtz (1990).

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